To provide a differential device wherein a companion flange nut does not easily loosen and the companion flange nut is prevented from rotating if loosened.

A companion yoke flange 9 has a rotation preventing socket 50. A plurality of teeth 51 are formed on the inner peripheral face of the rotation preventing socket 50, and the inner peripheral face of the rotation preventing socket 50 on which the plurality of teeth 51 are formed is fitted to the outer peripheral face of the companion flange nut 7.